A photodiode is a semiconductor device that converts light into current. In use, the current is generated when photons are absorbed in the photodiode. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas depending on the application.
The material used to make a photodiode is critical to defining its properties. This is mainly because only photons with sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents. Some materials used in photodiodes include metal wiring, silicon and germanium.
Crystalline germanium can be used as an optical detector; however if germanium is deposited in an amorphous form and it is not in contact with crystalline silicon it will require a long anneal at 450° C. to 550° C. in order to crystallize. This high temperature will result in many grain boundaries. Also, such high temperatures can destroy metal lines in the photodiode during the fabrication processes.